1. Field of the Invention
The present invention relates to an image acquisition method and transmission electron microscope.
2. Description of Related Art
Generally, a transmission electron microscope (TEM) is required to have an imaging system that has a small spherical aberration coefficient Cs for providing improved point resolution and a small chromatic aberration coefficient Cc for providing improved lattice resolution (see, for example, JP-A-11-135047).
A point resolution of an objective lens is defined by the first zero of a contrast transfer function (CTF) at the Scherzer focus. As the spherical aberration coefficient Cs decreases, the point resolution is improved. The first zero indicates a wave number at which the phase contrast transfer function at the Scherzer focus first crosses the axis of phase zero. The reciprocal of this wave number is the point resolution.
A contrast transfer function at the Scherzer focus is advantageous for providing improved point resolution and is important as a method for utilization in transmission electron microscopy that is applied to fields where tiny structures or objects of less than 1 nm are investigated and to the field of materials science where atomic resolutions better than 1 Å are required.
On the other hand, in the field of life science, objects or structures which are subjects of observation range widely in size from approximately 0.2 nm to as large as tens of nanometers. Information about these objects ranging widely in size is required. However, the optical system of a conventional transmission electron microscope cannot transmit information about low spatial frequencies and information about high spatial frequencies at the same time. It has been impossible to obtain information about objects ranging widely in size for the reason described below.
FIG. 10 is a graph showing a contrast transfer function at the Scherzer focus. The shown function has been derived as a result of a calculation performed under conditions where Cs=1.0 mm, Cc=2.0 mm, accelerating voltage V=200 kV, illumination angle α=0.1 mrad, and energy spread ΔE=0.8 eV.
FIG. 11 is a graph showing a contrast transfer function at a defocus of −2 μm. The shown function has been derived as a result of a calculation performed under conditions where Cs=1.0 mm, Cc=2.0 mm, accelerating voltage V=200 kV, illumination angle α=0.1 mrad, and energy spread ΔE=0.8 eV. Note that the negative sign “−” of the “defocus of −2 μm” denotes an underfocused condition.
As shown in FIG. 10, information about low spatial frequencies is little transferred at the Scherzer focus. Therefore, when structures or objects that are more than 1 nm in size are observed by phase contrast transmission electron microscopy, necessary information is not transferred at the Scherzer focus.
Accordingly, where relatively large microstructures are observed, the contrast transfer function is compressed by making the amount of defocus greater than the Scherzer focus value such that information about low spatial frequencies is obtained as shown in FIG. 11.
However, as the amount of defocus is increased, information about high spatial frequencies is impaired due to attenuation of the spatial envelope function as shown in FIG. 11. In this way, with the conventional optical system of a transmission electron microscope, it has been impossible to obtain information about low spatial frequencies and information about high spatial frequencies at the same time.